FIG. 1 is a block diagram illustrating a disk drive 100 in accordance with one embodiment of the present invention. As illustrated, the disk drive 100 is coupled to an external host computer 102 that uses the disk drive 100 as a mass storage device. It should be appreciated that the blocks illustrated in FIG. 1 are functional in nature and do not necessarily represent discrete hardware elements. For example, in one approach, two or more of the functional blocks within the disk drive 100 are implemented in software within a common digital processor.
With reference to FIG. 1, the disk drive 100 includes: at least one data storage disk 104, at least one transducer 106, an actuator arm assembly 108, a voice coil motor (VCM) 110, a read/write channel 112, an interface unit 120, a servo controller 122, and a disk drive controller 124. The disk drive 100 receives read and/or write requests from the host computer 102 and carries out the requests by performing data transfers between the at least one disk 104 and the host 102. In a preferred embodiment, the disk drive 100 includes multiple disks 104 in a vertical stack arrangement with one transducer 106 for each operative disk surface. Typically, both surfaces of each disk 104 will be operative for storing user data and, therefore, the disk drive 100 will include two transducers 106 for each disk 104. Single sided disk arrangements can also be used. The interface unit 120 is operative for providing an interface between the disk drive 100 and the host computer 102. During read and write operations, the interface unit 120 provides a communications path, including data buffering functions, between the host computer 102 and the read/write channel 112. In addition, the interface unit 120 is operative for receiving commands and requests from the host 102 and directing them to the controller 124. The controller 124 then carries out the commands by appropriately controlling the elements within the disk drive 100.
The voice coil motor (VCM) 110 is operative for controllably positioning the transducers 106 with respect to their corresponding disk surfaces in response to a control signal (e.g., icontrol) generated by the servo controller 122. Each transducer 106 is coupled to an integrated arm assembly 108 and move together under the influence of the VCM 110.
When performing a read or write operation, the controller 124 instructs the servo controller 122 to move one of the transducers 106 to a target track on a corresponding disk surface so that a data transfer can take place. The servo controller 122 then generates a control signal to move the identified transducer 106 from a present location to the indicated target track in a process known as a “seek” operation. Once the transducer 106 has arrived at the target track, the servo controller 122 enters a “track follow” mode during which the transducer 106 is maintained in a substantially centered position above the target track. The data transfer between the transducer 106 and the target track occurs during this track follow mode.
The read/write channel 112 is operative for, among other things, performing the data transformations necessary to provide communication between the host computer 102 and the disk 104. For example, during a write operation, the read/write channel 112 converts digital data received from the host computer 102 into an analog write current for delivery to one of the transducers 106. During a read operation, the read/write channel 112 provides the data transformations necessary for converting an analog read signal received from one of the transducers 106 into a digital representation that can be recognized by the host computer 102. The read/write channel 112 is also operative for separating out servo information read by a transducer and for directing this servo information to the servo controller 122 for use in positioning the transducer.
A lookup table 126 in drive 100 is operative for storing a write fault gate track threshold value for the transducers on disk 104 in the disk drive 100. The write fault gate threshold values are used by the disk drive 100 during write operations to determine when a corresponding transducer has moved too far off-track to reliably write data to the track. When performing a write operation, the disk drive controller 124 first retrieves a write fault gate value from the lookup table 126 corresponding to the transducer 106 associated with the write operation. The controller 124 then allows data to be written to the target track only when the corresponding transducer 106 is within a positional window about the target track that is defined by the retrieved write fault gate threshold value. The disk drive controller 124 monitors the position of the transducer 106 during the write operation to determine whether it is within the threshold window. As long as the transducer 106 is positioned within the window, the write operation is allowed to continue. If the transducer 106 moves outside of the threshold window, the controller 124 suspends performance of the write operation until a future time. Typically, the controller 124 will resume writing data on a next pass of the corresponding portion of the target track as long as the transducer 106 is properly positioned at that time. The controller 124 typically controls the writing of data to the target track using a write enable signal delivered to the read/write channel 112.
FIG. 2 is a diagrammatic representation of a simplified top view of a disk 104 having a surface 242 which has been formatted to be used in conjunction with a conventional sectored servo system (also known as an embedded servo system), as will be understood by those skilled in the art. As illustrated in FIG. 2, the disk 104 includes a plurality of concentric tracks 244a–244h for storing data on the disk's surface 242. Although FIG. 2 only shows a relatively small number of tracks (i.e., 8) for ease of illustration, it should be appreciated that typically many thousands of tracks are included on the surface 242 of a disk 104.
Each track 244a–244h is divided into a plurality of data sectors 246 and a plurality of servo sectors 248. The servo sectors 248 in each track are radially aligned with servo sectors 248 in the other tracks, thereby forming servo wedges 250 which extend radially across the disk 104 (e.g., from the disk's inner diameter 252 to its outer diameter 254). The servo sectors 248 are used to position the transducer 106 associated with each disk 104 during operation of the disk drive 100. The data sectors 246 are used to store customer data, which is provided by the host computer 102.
FIG. 3 illustrates a data storage disk 104 that is used to store digital data in a magnetic disk drive system. The disk 104 is substantially circular in shape and includes a center point 312 located in the center of the disk. The disk 104 also includes a plurality of tracks on a surface 314 of the disk 104 for storing the digital data. Ideally, each of the tracks is non-perturbed and ideally shares a common center 312 with the disk 104, such as ideal track 316 illustrated in FIG. 3. Due to system imperfections, however, actual written tracks on the disk 104 can be perturbed as compared to ideal tracks, such as non-ideal track 318 in FIG. 3. Consequently, transducer positioning is not as accurate on track 318 as it would be on an ideal track. Perturbation can be the result of incorrectly written servo information relative to the ideal track centerline (RRO), and it can also be due to perturbation of the transducer itself relative to the ideal track centerline (NRRO).
As illustrated in FIG. 3, the tracks on the disk 104 are each divided into a plurality of sectors 322. Each sector 322 is divided into a servo data portion and a user data portion (as described for FIG. 2). The servo data portion includes, among other things, information for use by the disk drive in locating a transducer above a desired track of the disk 104. When a host computer requests that data be read from or written to a particular track/sector of the disk 104, the transducer must first be moved to the track and then must be positioned at a predetermined location with respect to the centerline of the track before data transfer can take place. The disk drive uses the information stored in the servo data portion of each sector to first locate the desired track and to then appropriately position the transducer with respect to the centerline of the desired track.
FIG. 4 illustrates a typical servo pattern 424 stored within the servo portion 248 of a sector 322 for use in centering a transducer 106 on a desired track. The servo pattern 424 includes a plurality of servo bursts 426–432 that define the centerlines 434–438 of the tracks of the disk 104. The bursts 426–432 are divided into A bursts 426, 430 and B bursts 428, 432 that are each approximately a track-width wide and which alternate across the disk surface. The boundary between an A burst and an adjacent B burst (e.g., A burst 430 and B burst 428) defines the centerline (e.g., centerline 436) of a track on the disk. To center the transducer 106 using the A and B bursts, the transducer 106 is first moved to the desired track during a seek operation and, once there, is allowed to read the A and B bursts on the desired track. The signal magnitudes resulting from reading the A and B bursts are then combined (such as by subtracting the B burst magnitude from the A burst magnitude) to achieve an error signal, known as the position error signal (PES), which is indicative of the distance between the center of the transducer 106 and the centerline of the desired track. The PES signal is used by the disk drive to change the position of the transducer 106 to one that is closer to the desired (centered) position. This centering process is repeated for each successive sector on the track until the requested read/write operation has been performed in the appropriate sector 322 of the disk 104. It should be appreciated that other schemes for storing servo information on the magnetic media (such as schemes using zones, constant linear density (CLD) recording, split data fields, and/or hybrid servo) can also be used in accordance with the present invention.
The A and B bursts 426–432, as well as other servo information, are written to the surface 314 of the disk 104 using a servo track writer (STW) after the disk 104 is assembled into the disk drive during the manufacturing process. It is these A and B bursts which define the location of the written tracks on the disk 104. That is, on a non-ideal track (such as track 318 of FIG. 3) the A and B bursts are written such that the centerline of the track is not smooth, but rather is perturbed; this is the source of RRO. Further, a transducer can be made to position itself in a window positionally relative to the path of an ideal track by adding an appropriate offset value to the PES signal. Offset values, relative to the known RRO, may be used to modify the controller commands to the actuator and correct the RRO as the transducer follows the track. RRO correction values are stored within the servo portions 248 of each sector 322 of the disk for use in positioning the transducer 106 in an approximation of ideal track path, such as 316, during track following operations.
As above mentioned, when a transducer moves off-track during a write operation, there is a chance that the transducer might inadvertently write data on or near an adjacent track, thus corrupting the data written on the adjacent track. In addition, the data that is written off-track by the transducer may be difficult or impossible to read during a subsequent read operation on the present track due to its off-track position. Thus, an off-track threshold value previously identified as a write fault gate is typically defined on a disk drive that indicates an off-track transducer position beyond which the write operations will be suspended. If the transducer goes beyond the limits of the write fault gate threshold during a write operation, the write operation is suspended until the transducer again comes within the specified positional window about the target track.
In the prior art, the write fault gate threshold was determined during disk drive development based upon collected (worst case) off-track threshold data and estimates of transducer positioning error. Using a 3-sigma statistical distribution of the estimates of the transducer positioning error, a write fault gate threshold was set to an approximate value of value of 1.3 times the 3-sigma value of the position error at the worst case of the stroke, i.e., at the outer diameter (OD).
The single write fault gate threshold value thus derived was then used for all transducers within all drives in a production run. During disk drive tests, if the off track capability of the transducers in a particular drive were all within a specified range and the measured position error of the drive was within a corresponding range, the disk drive would pass certification limits. It would be assumed that the write fault gate threshold programmed into the drive would be sufficient to prevent adjacent track data corruption and unreadable off-track data. If the off track capability of a transducer was not within the specified range, the transducer would not be used in a disk drive. Similarly, if a particular disk drive displayed greater than a predetermined position error, the drive also would not be used. As can be appreciated, the greater the number of units that are left unused during the manufacturing process, the greater the overall manufacturing costs.
In an attempt to overcome the shortcomings of specifying a single off-track capability/write fault gate value for an entire production run of disk drives, unique off-track capability/write fault gate values were generated for individual disk drives during the manufacturing process. The write fault gate threshold values were determined based on the measured off track capability of each of the transducers actually within each drive as well as a positioning error associated with that disk drive.
Because the write fault gate values were variable from drive to drive, transducers that were previously discarded as not falling within a predetermined off track capability range could be used as long as they occur in a drive having lower positioning errors. Similarly, drives having a large positioning error can be used if paired with transducers having superior off track capability. In this manner, manufacturing yields were increased without compromising disk drive performance.
Accordingly, in the recent prior art, a separate write fault gate threshold value was generated for each of the transducers within a manufactured disk drive. In one approach, a look up table is provided within the disk drive for storing the write fault gate values used by the drive. An appropriate value is retrieved from the look up table for each write operation performed by the disk drive. These write fault gate values are normally generated during the test procedure as part of the manufacturing process. Again, the prior art values are based on the position error at the worst case condition.
While the above described method of generating separate write fault gate values for each of the transducers in a disk drive permits matching transducers to a drive, enabling more drives to be certified for shipment, the write fault gate threshold values for the transducers are still limited by the minimum write fault gate threshold attributable to the position error at the worst case, i.e., at the outer diameter. Factors at the OD, such as air turbulence, radial distance from the inner diameter, etc., increase the threshold values of the write fault gates. At present, the write fault gate thresholds associated with a disk drive, even those providing write fault gate thresholds for each transducer, are a constant across the stroke of the actuator. However, tracks along the inner diameter, less subjected to the errors such as flutter, turbulence, vibration, etc. introduced into the disk along the outer diameter, may utilize smaller write fault gate thresholds but are still constrained to the same write fault gate thresholds as at the outer diameter tracks.
Accordingly, it would be advantageous to provide a method to accommodate the decrease in the write fault gate threshold requirements on the tracks as the transducer moves along the stroke toward the inner diameter of the platter, permitting a reduction in the track width and an attendant increase in track density.